1. Field of the Invention
The present invention relates to a drive circuit for an ultrasonic motor and a drive control method for an ultrasonic motor. More particularly, this invention is concerned with a drive circuit for an ultrasonic motor and a drive control method for an ultrasonic motor that can control the driving frequency for an ultrasonic motor which varies depending on temperature, load, or the like so as to achieve optimal lock-on.
2. Description of the Related Art
Various drive circuits for an ultrasonic motor have been proposed in the past. For example, Japanese Patent Laid-Open No.59-204477 has disclosed a technological means, wherein based on the fact that the voltage value of a monitor signal becomes maximum at a resonance frequency of an oscillator in an ultrasonic motor, the voltage value of the monitor signal is monitored, and an ultrasonic motor is driven at a resonance frequency or frequencies in a band slightly higher than the resonance frequency in accordance with the monitored voltage value.
The present applicant has disclosed in Japanese Patent Laid-Open No.63-234881 a technological means that uses a signal sent from a frequency divider, which generates drive voltage for an ultrasonic motor, to detect a phase of a monitor signal, and, similarly to the disclosure in Japanese Patent Laid-Open No.61-251490, drives the ultrasonic motor at a resonance frequency or frequencies in a band slightly higher than the resonance frequency in accordance with the detected phase of the monitor signal.
In the disclosure of Japanese Patent Laid-Open No.59-204477, as shown herein in FIG. 10, the voltage V of a monitor signal is monitored during an interval between frequencies f1 and f2; that is, a control frequency range, and then frequency control is carried out. The voltages of the points P.sub.A1 and P.sub.A2 on the monitor signal associated with the frequencies before and after a resonance frequency fR, for example, frequencies fA1 and fA2 in FIG. 10 are the same or V.sub.A. Therefore, when the voltage of the monitor signal is V.sub.A, unless the frequency f is shifted to a higher or lower frequency in order to look at the tendency of the voltage of the monitor signal, it is uncertain in which direction a driving frequency should be shifted. This means that it takes much time to lock onto an optimal frequency. It is very hard to control a frequency precisely and quickly.
According to the disclosures of Japanese Patent Laid-Open Nos.61-251490 and 63-234881, as shown herein in FIG. 11, since the direction in which a phase .theta. of a monitor signal shifts becomes constant around the resonance frequency f.sub.R, the direction in which a frequency should be shifted can be determined as soon as the phase .theta. of the monitor signal is detected. This control method makes it possible to perform frequency lock-on more rapidly and precisely than the method based on the voltage of the monitor signal disclosed in Japanese Patent Laid-Open No.59-204477.
As shown in FIG. 11, in the vicinity of the resonance frequency f.sub.R, the phase .theta. of the monitor signal has a relationship of increasing in the lagging direction with increase with the frequency f; that is, when the frequency f gets higher, the phase of the monitor signal lags further behind. However, when the frequency f is made still higher and is shifted to a higher frequency than a frequency f.sub.p in FIG. 11, the phase .theta. of the monitor signal changes toward leading. Therefore, even if an attempt is made to control a frequency within a normal control frequency range of f1 to f2 and to converge the frequency to the state of a point PC1; that is, a frequency f.sub.C1, the frequency may be converged to the state of a point P.sub.C2 ; that is, a frequency f.sub.C2, by mistake.
Therefore, when an ultrasonic motor is operated at a frequency, especially, in a stable band higher than a resonance frequency, a setting range between frequencies f1 and f2 is limited or a target phase value is restricted.
However, when the foregoing ultrasonic motor is incorporated in a camera lens and operated at varying temperatures from a low temperature to a high temperature, or when the lens is subjected to a drastically varying load because of an accessory or the like attached to the distal end thereof, the control frequency range of f1 to f2 must be reserved widely. In this case, the disclosures proposed in Japanese Patent Laid-Open Nos. 61-251490 and 63-234881 cannot lock onto an optimal frequency in a stable and highly efficient manner.